In air conditioners air flowing through an evaporator rejects heat to the evaporating coil and simultaneously condenses moisture on the heat transfer surface of the same coil. However, in high ambient humidity, dehumidification by air conditioners is often not sufficient.
The most popular way of dehumidification requires the installation of a dehumidifier in addition to an air conditioner. In the US the sale of portable dehumidifiers jumped from close to none in 1991 to more than 1,000,000 in 1998. Dehumidifier gives consumers an advantage to control independently both parameters of indoor air: temperature and relative humidity. A thermostat controls the operation of the air conditioner depending on the room temperature and a humidistat controls the operation of the dehumidifier depending on the humidity in the room. However, this technology consumes excessive amount of energy. First, a dehumidifier itself consumes energy to run a compressor and a fan. Second, unlike an air conditioner where the condenser rejects heat to ambient, in a dehumidifier the combined energy of both the compressor and the fan goes back to the room. To offset the influx of this energy the air conditioner should have extra capacity and spend extra energy.
Several attempts have been made to achieve sufficient dehumidification of conditioned air without an extra dehumidifier. Some designers use oversized air conditioners to reduce the evaporating temperature and increase moisture condensation. However, oversized air conditioners with low evaporating temperature reduce air temperature beyond comfortable level. To overcome this beaters to reheat air after evaporators are used. The heaters can be independent of air conditioners, e.g., electrical or heated by steam or hot liquid, or they can reclaim heat from the system condensers. However, the use of such strategies is also energy costly. Much like with an extra dehumidifier, an air conditioner should offset energy generated by the heater. Besides, "independent" heaters consume extra energy, while heaters using "reclaimed" heat require extra connections and extra coils and can not be disconnected when there is no need for reheating air.
The best alternative is to use a properly sized air conditioner to cool and dehumidify indoor air. It is widely recognized, however, that the larger evaporating coil the higher evaporating temperature of refrigerant, and the higher efficiency and capacity of the air conditioner, and the lower temperature of air exiting the evaporator. Still, moisture condensation depends mainly on the temperature of heat transfer surface of the evaporator. Besides, relative humidity of air leaving the evaporator is also a function of the air temperature. Thus, large evaporators can lead to excessive relative humidity of conditioned air that in turn causes damp and mould in the room. At the present time designers often limit the size of evaporators especially for climate with high humidity. On the other hand, the undersized evaporator reduces efficiency and capacity of the air conditioner.
To overcome this some designers use a method that involves heat pipe technology. See, for example, U.S. Pat. Nos. 5,333,470 and 5,448,897. Such design adds two additional heat exchangers to the evaporator: one is the "precooling" coil upstream of the evaporator, another is the Preheating coil downstream of the evaporator. Two coils are filled with phase change medium and connected to each other the way that the coil upstream of the evaporator picks heat from the incoming air and pumps this heat to the coil downstream of the evaporator and to outgoing air. Thus, the temperature of incoming air and the temperature of the heat transfer surfaces of the evaporator are decreased, which causes additional condensation and reduction in absolute humidity of the air. Because the heat from incoming air increases the temperature of air exiting the coil downstream of the evaporator, relative humidity of exiting the air conditioner air is reduced considerably. However, installation and operation of heat pipes generally involves notable expenses. In addition, such systems lead to an excessive pressure drop in air stream because there are two extra heat exchangers. In case there is no need in relative humidity reduction there is some extra complication involved in disabling of the heat pipe.
There is also a solution involving the subcooling technology. See, for example, U.S. Pat. No. 5,689,962 and Carrier Corporation "NoistureMiser". According to U.S. Pat. No. 5,689,962 a subcooling coil is installed in an air passage downstream of the evaporator. Leaving the condenser hot liquid refrigerant expands in a pressure reduction device, then flows to the subcooling coil, rejects heat from refrigerant to air cooled in the evaporator, recondenses, and goes to an expansion device and then to the evaporator. In the evaporator liquid refrigerant evaporates absorbing heat from air. Because refrigerant is preliminary subcooled, capacity of the evaporator increases and the temperature of its heat transfer surface goes down. Same as with the heat pipe technology it causes additional condensation and reduction in absolute humidity of air. Because the temperature of air exiting the subcooling coil after the evaporator is increased, relative humidity of air is reduced. This design is relatively simple and can practically achieve the same effect as heat pipes. However, the system does not have any means to regulate moisture condensation. In case of air with initial low humidity, the subcooling bringing an extra load on the evaporator with reduction of the evaporating temperature causes the reduction in capacity and efficiency compared to the conventional system. Furthermore, if inflow of fresh air is not sufficient and humidity is low, excessive dehumidification can bring relative humidity of air below 50%, which is beyond comfortable conditions. In addition, at the initial moment just after the air conditioner starts there is no cooled air after the evaporator. Subcooling coil itself is also warm. Thus, the refrigerant expanded in the pressure reduction device will not be recondensed completely. This causes excessive pressure drop in the expansion device, which leads to a very low evaporating temperature and reduction in capacity. That, in turn, increases the transition time and efficiency of the system.
Carrier's "MoistureMiser" also uses an extra coil for subcooling to reduce the temperature of the evaporating surfaces and increase moisture condensation. Unlike the system of the U.S. Pat. No. 5,689,962 the extra coil in the "NoistureMiser" can be bypassed and air conditioner works without subcooling. However, this extra coil may often work not as a subcooler but as an additional part of the condenser, especially in the small size systems with the fixed expansion device. When the extra coil works as a subcooler it must be filled with liquid refrigerant. Thus, additional amount of refrigerant is required in the system. In addition, the extra coil adds pressure drop in air stream, which reduces the airflow rate.